Iodine Adsorption in a Redox-Active Metal–Organic Framework: Electrical Conductivity Induced by Host−Guest Charge-Transfer

We report a comparative study of the binding of I2 (iodine) in a pair of redox-active metal–organic framework (MOF) materials, MFM-300(VIII) and its oxidized, deprotonated analogue, MFM-300(VIV). Adsorption of I2 in MFM-300(VIII) triggers a host-to-guest charge-transfer, accompanied by a partial (∼30%) oxidation of the VIII centers in the host framework and formation of I3– species residing in the MOF channels. Importantly, this charge-transfer induces a significant enhancement in the electrical conductivity (Δσ = 700000) of I2@MFM-300(VIII/IV) in comparison to MFM-300(VIII). In contrast, no host–guest charge-transfer or apparent change in the conductivity was observed upon adsorption of I2 in MFM-300(VIV). High-resolution synchrotron X-ray diffraction of I2@MFM-300(VIII/IV) confirms the first example of self-aggregation of adsorbed iodine species (I2 and I3–) into infinite helical chains within a MOF.

The solution was degassed under Ar for 0.5h and heated at 483K for 3 days to produce a green microcrystalline powder. The product was separated by filtration, washed with DMF and dried in air (80% yield).

Synthesis of MFM-300(V IV )
MFM-300(V III ) was heated under a pure oxygen flow at 150 o C overnight using a tube furnace at a ramping rate of 1 o C min -1 . MFM-300(V IV ) was obtained as black solid and stored under N2 in a glovebox.

Scanning Electron Microscopy
SEM measurements were undertaken on a Quanta 650 at a working voltage of 20kv with a scale bar up to 1 micron.

Iodine Adsorption and Cycling Test
MFM-300(V III ) and MFM-300(V IV ) were synthesized following previously reported methods, 1 the assynthesized MFM-300(V III ) and MFM-300(V IV ) were exchanged with acetone for a week. Complete activation was achieved by heating the acetone-exchanged samples under vacuum (10 −5 mbar) for 12 h at 150 o C. The activated sample was then inserted into a Schlenk flask which contains a vial with an excess of solid I2. Pure N2 (> 99.999%) was slowly dosed into the flask to reach at atmospheric pressure.
The flask was then heated at 80 o C for 2 days to allow the full adsorption of I2 into the desolvated MOFs, a small portion of the samples were taken out for further analysis and the rest of them are still kept in the Schlenk flask and used for further desorbed and cycling experiments.

Thermogravimetric−Mass Spectrometry (TGA-MS) Analysis
Thermogravimetric analysis (SDTQ600 TA Instruments company) coupled with mass spectrometry (Hiden DSMS analyzer) was used to calculate the uptake of adsorbed I2 molecules within MFM-300 (V III ) and MFM-300(V IV ). Samples were heated from room temperature to 600 °C at a heating rate of 5 degree min −1 under a flow of air. The uptake of I2 in MFM-300(V) was determined by the weight loss of adsorbed I2 confirmed by mass spectrometry and detection of the characteristic peak at 127 (I·).

High Resolution Powder X-ray Diffraction Data
High resolution synchrotron powder X-ray diffraction (PXRD) data were collected at Beamline I11 of Diamond Light Source using multi-analysing crystal-detectors (MACs) and monochromated radiation [λ = 0.824677(10) Å]. The powder samples were loaded into capillary tubes of 0.7 mm diameter and the data collection was carried out at room temperature.

300(V IV )
The structural model of MFM-300(V III ) was used as a starting point for Rietveld refinements, which were carried out with Topas Academic 5 program (http://www.topas-academic.net/). The organic linkers were modelled as semi-rigid bodies where selected bond distances, bond angles and torsion angles could be refined. Crystallographic positions of adsorbed I2 were found by sequential difference Fourier map calculations and electron density peaks analysis starting from the activated crystal structure.
The final Rietveld refinements included crystal cell parameters, background and profile coefficients, all atomic positions and occupancies for iodine atoms, which were constrained to be the same value for the atoms within each I2 molecule. Crystallographic parameters from the final Rietveld refinement are summarized in Table S1.

X-ray Photoelectron Spectroscopy
Measurements of the I 3d core level were performed using an Axis Ultra Hybrid spectrometer (Kratos Analytical, United Kingdom), using Al-Kα radiation (1486.6 eV, using 10 mA emission at 250 W).
Binding energy calibration was performed using the C 1s photoelectron peak from adventitious hydrocarbon (284.8 eV).

Electrical Conductivity Test
Electrical conductivity measurements were conducted using a Solartron Analytical Modulab XM Materials Test System (Solartron Analytical, Farnborough, UK) over a frequency range from 0.1 Hz to 1 MHz. Ag paste electrodes were coated on both faces of the pellets of MOF. The impedance data were corrected for sample geometry (thickness/area) and analysed using the commercial software package ZView (Version 3.5e, Scribner Associates Inc., USA).

Electrochemical Measurements
Electrochemical measurements were conducted in a standard three-electrode glass cell connected with a CHI760e electrochemical workstation. The three-electrode electrochemical cell comprised of a glassy carbon (3 mm) working electrode, which was modified with MOF. A carbon rod was chosen as the counter electrode, and a saturated calomel electrode (SCE) or Ag/AgCl electrode was selected as the reference electrode. 10 mg of MOF was dispersed in 1 mL of iso-propanol (970 μL)/Nafion (30 μL) by sonication to form a homogeneous ink and then 6 μL of the ink was loaded onto the working electrode over an area of ~0.0706 cm 2 ). Electrochemical impedance spectroscopy (EIS) measurements were carried out from 100000 Hz to 0.1 Hz.